With carbon offset schemes, an organisation or an individual compensates for the carbon emissions they are going to make by paying for work which reduces the amount of carbon dioxide in the atmosphere by an equivalent amount.

However, that carbon removal takes place over time - twenty years or more with a planted tree - and many carbon offset schemes do not verifiably produce the outcome the promoters sell.

Companies such as Dell, with its Plant A Tree For Me and Plant A Forest initiatives, are big users of carbon offset schemes.

"Tree-planting schemes are particularly problematic and should be ruled out of any offset scheme," FOE states. "Large-scale plantations have decreased biodiversity, displaced people and caused social disruption. Doubts have recently been cast on the contribution of trees outside the tropics in reducing CO2 levels and the science is uncertain."

Google intends to be carbon-neutral by the end of this year and adds carbon offsets to the raft of measures it is taking. But such measures don’t include ditching the corporate jets its founders and its CEO maintain.

“Carbon offsetting should be a measure of last resort, after steps have been taken to cut emissions,” said Tony Juniper, a director at FOE.

Companies such as Dell and others will no doubt say that offsetting is being used as a last resort.

Google states: “For the ... piece of our (carbon) footprint that we can’t eliminate through greater energy efficiency and renewable energy generation, the only way to become carbon neutral in the short term is through carbon offsetting. ... we believe that offsets can offer real, measurable, and additional emissions reductions that allow us to take full responsibility for our footprint today.”

But Juniper remains unconvinced. “Offsets can even encourage businesses and people to continue with (or even increase) unnecessary polluting activities, promoting the mindset ‘I’ve offset so it’s OK to fly’, ‘I’ve offset so it’s alright to drive to work’ etc,” he said.

The environmental pressure group believes the purchase of offsets should only be considered after comprehensive measures to avoid and cut emissions across activities. It feels that in the many moves to counter global warming there is only a small role for offsetting schemes. It also thinks that all such schemes need to be properly regulated.